The invention relates to a power wrench including a mechanical override type release clutch as well as means for detecting the release of the clutch to detect the attainment of a predetermined target torque level when tightening screw joints.
In one type of prior art power wrench, as shown for instance in U.S. Pat. No. 5,201,374, a predetermined release torque level is detected by a mechanical device connected to one of the clutch halves for transferring a release movement to a power shut-off device. This mechanical type of target torque detection suffers from the drawback of an undesirable mechanical influence on the clutch operation in that the release torque level to some extent is dependent on the frictional resistance in the release detecting mechanism.
In another type of prior art wrench, as shown for instance in U.S. Pat. No. 5,868,208, the attainment of a predetermined target torque level is indicated by a micro switch indicating a release movement of one of the clutch parts. The switch is usually connected to an electric control unit for initiating power shut-off as a clutch release is indicated. This type of clutch release sensing device is disadvantageous in that it is exposed to mechanical wear and therefore has a limited service life.
In still another type of prior art wrench, the clutch release is indicated by a Hall-element located close to the clutch for detecting the release movement of one part of the clutch and delivering an electric signal in response thereto. This type of release detection, however, requires a specific clutch design providing an extended or amplified release movement for ensuring a proper activation of the Hall-element.
In order to avoid the problems and drawbacks of the above described previous devices for detecting the target torque level and clutch release point, the invention provides a technique which does not have any influence mechanically or otherwise on the release movement of the clutch and which requires very little space and which does not require any special clutch design.
A general object of the invention is to provide a technique whereby the actual release of the clutch is detected. This is in contrast to most other techniques which detect a change in position of a moving part of the clutch, which might not correctly reflect the fact that the clutch has actually released. Especially at low output shaft rotation speeds, the inertia of the motor and gear mechanism might not be enough to ensure clutch release after a specific position of the clutch has been detected.
So, the main object of the invention is to provide a power wrench in which the attainment of a predetermined target torque is detected as a release sound of an override type torque responsive release clutch that is provided comprised in the power wrench.
Further objects of the invention will appear from the following description and claims.